Two outstanding questions in biology are how stem cell self-renewal is controlled and how a cell with multiple differentiation options becomes restricted to a single developmental fate. The hematopoietic system is an excellent model to study these questions owing to its derivation from a pool of stem cells that maintains production of at least eight distinct cell lineages throughout life. This model is also clinically relevant because HSCs are the foundation for bone marrow transplantation and mistakes in this developmental program can lead to severe disease such as immune deficiency or leukemia. Therefore, to design effective therapies to intervene in disease and to predict the effects of therapeutic intervention on hematopoietic development we need to gain a mechanistic understanding of the factors that control hematopoiesis. The E protein transcription factors are essential regulators of cell fate in vertebrates and invertebrates and interact with multiple proteins implicated in hematopoietic stem cell (HSC) development and function. Nonetheless, a role for E proteins in HSCs or in early hematopoietic cell fate decisions has only recently been demonstrated. We have determined that the E proteins encoded by the E2A gene are required for proper maintenance of HSCs and they support the initial specification of multipotent progenitors toward the lymphoid lineages. In Aim 1 of this application we propose experiments to test the hypothesis that E2A proteins promote HSC self-renewal during regeneration of bone marrow following transplantation. We will also test the hypothesis that E2A- deficient HSCs fail to home appropriately to the HSC niche and/or fail to be retained within the niche. In Aim 2 we will test the hypothesis that lymphoid specification requires E protein homodimers and we will determine the mechanisms controlling homodimers formation. In addressing these hypotheses we will gain important insight into the molecular mechanisms underlying hematopoietic stem cell function and differentiation toward the lymphoid lineages. Moreover, since E protein antagonists can be modulated by signaling pathways, for example transforming growth factor-21 or bone morphogenic proteins, our results may reveal mechanisms by which these pathways can influence hematopoiesis.